Study of the propagation centre in the anionic polymerization of (meth)acrylic monomers, 5. Nuclear magnetic resonance study of the model dimer in tetrahydrofuran

Di-tert-butyl 2-lithio-2,4,4-trimethylglutarate ( 1 ) as a model of the living dimer of tert-butyl methacrylate was studied in deuterated tetrahydrofuran (THF-d₈) solution by ⁶Li, ⁷Li, ¹H and ¹³C NMR. In agreement with earlier observations from infrared spectroscopy and with quantum chemical ab initio and semiempirical self-consistent field calculations, 1 is shown to have a strong tendency to an intramolecular coordination of Li to the penultimate ester carbonyl group. The competing linear form of 1 (with uncoordinated penultimate ester group) is shown to exist due to its stabilization by self-aggregation to form a dimer of 1 . In contrast to the monomeric form of 1 , the dimer does not bind THF into a specific solvate. Given the entropy-driven solvation at low temperatures, the occurrence of the dimeric aggregate is enhanced by higher temperature and concentration of 1 . Both the cyclic and linear form of 1 are shown to exist in at least two different conformers and various solvation states. Their mutual exchange is several orders of magnitude slower than the anionic propagation reaction of tert-butyl methacrylate.